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I want to connect two external displays to my laptop running Ubuntu 18.04 LTS.

Setup: My laptop (Dell XPS 17 l702X) has an internal graphics card (Intel) and a dedicated graphics card (NVidia GeForce GT 555M). It also has DisplayPort and HDMI slot. The DisplayPort is connected to the Intel card and the HDMI port is connected to the NVidia card. I am using the proprietary NVidia drivers (nvidia-driver-390).

Problem: If I use HDMI only, I can get an output on the external monitor. Otherwise, I only get an output on the DisplayPort monitor, never on both of them. In the Display settings, only one monitor shows up.

Efforts: I also tried to switch between graphic cards using prime-select and the "NVidia X Server Settings" tool and also the open X.Org drivers without any success.

Question: How is it possible to make both displays run under Ubuntu?

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That is cause due the laptop screen is running in the low performance intel graphic, you could try to bypass completely the intel graphic and run also the laptop screen under nvidia graphic. You can be done in a couple of way, first (if possible) disable dinamic graphic from bios or use xrandrd.

Here a guided solution, it is from debian wiki but should work out also in Ubuntu: https://wiki.debian.org/NvidiaGraphicsDrivers/Optimus

Update

Another solution a bit more difficult is to configure manually the xorg server as you want, by a quick research I find out this, may could help you: https://superuser.com/questions/117239/how-can-i-get-multiple-video-cards-to-work-on-linux

you have to define two Screen, and assign each Screen to desired device

  • Sounds promising. I'll try it out and report my results. Thank you. – Green Jan 4 at 17:52
  • Good luck, I'm always "fight" with the xorg.conf, I was really happy the day distribution can perform decent video configuration "on the fly" which supersedes, mostly, the xorg.conf – AtomiX84 Jan 4 at 18:49
  • Thanks for your answer. By reading the debian wiki entry, I've got the idea that selecting the NVidia card as primary card has to be the key to success here. Then, I've just tried to set the NVidia card as primary and then reboot. In fact, before I didn't tried to reboot, instead always logout/login as suggested in the dialog box that appears after I set the NVidia card as primary. Apparently, the reboot was the missing puzzle part. – Green Jan 6 at 9:56
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By accident, I figured it out now. The key is use the NVidia card instead of the Intel. I did it via "NVidia X Server Settings", then reboot. Before reboot, I turned off the display that is attached to the Intel card - just to be sure. After reboot, Ubuntu automatically registered the laptop screen and the screen attached to the NVidia card. Then, by simply turning on the third display, Ubuntu recognized that one as well and now, I can use three screens.

I guess, the enabling factor was simply to reboot instead of just to logout and login again after selecting the NVidia card.

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